This invention relates to the gasification of coal and similar carbonaceous solids and is particularly concerned with a method for maintaining a relatively high gasifier bed density in a fluidized bed gasification process.
The formation of agglomerates is a problem frequently encountered in the gasification of caking coals. This is caused by plastic properties which develop when such coals are subjected to temperatures above their softening point. Upon reaching this point, generally between about 370.degree. C. and about 480.degree. C., the coal particles begin to swell and then deform due to the formation of bubbles during devolatilization. As the temperature increases, deformation becomes more severe, the coal becomes plastic and sticky, and may eventually become fragile. The sticky particles tend to agglomerate and form coherent solid masses which reduce gas permeability and tend to block the reactor and the reactor feed lines.
Several methods have been devised to alleviate agglomeration problems encountered when caking bituminous coals are gasified. One such method proposed in the past is to pretreat the raw coal by contacting it with air at relatively high temperatures prior to passing the coal into the gasification reactor. Past studies have indicated that treatment of raw coal in such a fashion tends to destroy its caking properties and thereby prevents it from agglomerating during gasification. It has been noted that such pretreatment procedures have pronounced disadvantages in that they are either expensive, result in the loss of valuable volatile constituents from the coal or reduce the amount of methane that can be produced from a given amount of coal. Thus, these pretreatment procedures are undesirable for use in a gasification process. In lieu of pretreating raw coal with air at high temperatures, it has been suggested in the past that coal be treated with a dilute aqueous solution of sodium hydroxide at room temperature in order to decake the coal. Past studies have shown that raw coal treated with aqueous solutions of sodium hydroxide exhibit a free-swelling index in the range from about 1 to 2 and is therefore relatively noncaking at atmospheric pressure.
Because of past teachings that sodium hydroxide tends to decake coal, it was felt that agglomeration would not be a problem in catalytic gasification of caking coals if the alkali metal catalyst was added to the raw coal prior to introduction of the coal into the gasifier. Indeed, it has been found in the past that when coal is impregnated with alkali metal compounds and gasified in a fluidized bed at relatively low pressures, agglomeration problems are substantially obviated. It has now been surprisingly discovered, however, that when fluidized bed catalytic gasification is carried out at higher pressures, the density of the resultant fluidized bed of char particles is very low, in some instances as low as 80 kg/m.sup.3. Such low bed densities result in a substantial reduction in the amount of coal that can be processed in a given gasifier and therefore substantially decrease the amount of product gas that can be produced in the process. In order to convert more coal into gas per unit of time, it would be necessary to utilize a much larger gasifier or to employ multiple gasifiers, procedures that would result in a substantial increase in the investment cost of a commercial plant and the resulting price of the product gas.